The invention relates to a stereo pilot signal eliminating circuit, and more particularly to a stereo pilot signal eliminating circuit for use in an AM stereophonic receiver.
In a known AM stereophonic system, transmission of AM stereophonic broadcasts is carried out by phase-modulating a carrier signal with a sub-signal which corresponds to the difference between the right and left channel signals after which the signal thus initially modulated is subjected to amplitude modulation with a main signal which is a sum signal of right and left channel signals. In this case, when the carrier wave is subjected to phase modulation, a low frequency (5 Hz or 25 Hz) stereo pilot signal, which is indicative of the presence of stereophonic signals, is superposed on the carrier signal together with the sub-signal.
In an AM stereophonic receiver adapted to receive such AM stereophonic broadcast signals, the main signal is detected by an envelope detector which extracts the variations in the amplitude component of the carrier signal. After removing the variations in the amplitude component of the carrier signal with a limiter, the sub-signal is detected by a sub-signal detector circuit. Then, both detected signals are multiplexed and the right and left channel signals are separately outputted.
In the above-described system, since the output of the detector circuit for the sub-signal contains the stereo pilot signal, the pilot signal is present in both of the right and left channel signal components when the detected outputs of the main signal and the sub-signal are multiplexed in a matrix circuit. This causes a deterioration of the output characteristics in the right and left channels. In order to solve this problem, a method has been employed in which a pilot signal eliminating filter is connected at the output of the matrix circuit. However, this method is disadvantageous in that the circuit arrangement is complicated and the response characteristics in the lower frequency ranges of the respective channel outputs are lowered.
Accordingly, an object of the present invention is to provide a pilot signal eliminating circuit for use in an AM stereophonic receiver in which the stereo pilot signal is cancelled by a simplified circuit arrangement and the frequency characteristics of the channel outputs are not substantially degraded.